Magnification Modes of Operation for Common Axis FOV Switching and Image Roll

ABSTRACT

A reflective afocal switching assembly permits variable fields of view while at the same time providing a common axis and mechanism to achieve an optical de-roll of the image. This complex arrangement provides a relatively large change in magnification for an all-reflective optical system than can image over 0.4-12.0 micron spectrum.

REFERENCE TO RELATED APPLICATIONS

This is a divisional patent application of copending application Ser.No. 15/786,630 filed Oct. 18, 2017, entitled “Multiple Field of ViewReflective Afocal Assembly with Common Axis for FOV Switching and ImageRoll.” The aforementioned application is hereby incorporated herein byreference.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, sold,imported, and/or licensed by or for the Government of the United Statesof America.

FIELD OF THE INVENTION

This invention is applicable to the field of optics, particularly inregards to a multiple field of view reflective afocal assembly withcommon axis.

BACKGROUND OF THE INVENTION

Off-axis reflective optical systems, both focal and afocal, have founduse especially in applications where a broad spectral coverage and highthroughput is required. Several examples of off-axis reflective opticsare found in the prior art, including U.S. Pat. No. 6,274,868. Theoff-axis nature of the mirror elements eliminates the centralobscurations found in conventional on-axis design forms such as thestandard Cassegrain telescope. The advantages of the off-axis opticalsystem forms are offset by the fact that the mirror alignments must beextremely precise, as there are no rotational degrees of freedom to takeadvantage of during manufacture. The required positional precisionunfortunately also precludes the prior art techniques of providingmanual or automatic mechanical adjustments of the positions of themirrors in order to achieve effects such as focusing, zooming, etc. Inthe case of an off-axis reflective afocal optical system, it isgenerally not economically feasible to provide a mechanical adjustmentof the existing mirror system which alters the magnification powerwithout reducing the optical image quality. One prior art solution isdescribed in U.S. Pat. No. 5,477,395 wherein two pre-aligned afocalassemblies are nested along a common rotational axis, such that one orother assembly can be rotated into place over the pupil in order toselect a magnification. A disadvantage of U.S. Pat. No. 5,477,395however, is that the described axis of mechanical rotation for the FOVswitch, either horizontally or vertically, does not coincide with theoptical axis through the pupil of the system. Because of thatlimitation, the mechanical switching system cannot serve a dual purposeto adjust image roll.

Image roll is a phenomenon common in aircraft sensor systems, where theaircraft pitch, yaw, and roll relative to the earth can cause the sensorprojection on the ground to rotate. In many cases, the pilots prefer tohave a “de-roll” capability to offset the variable effects of aircraftattitude. For navigation use, de-roll can be used to ensure the earth'shorizon as seen by the sensor remains in same position as seen out thewindow. In digital mapping applications, de-roll can be used to ensureeach ground sample image is aligned symmetrically with the nextsequential sample image so that they can be stitched together to form acomposite map. In the prior art for nested multiple field of viewafocals, a de-roll capability would require a second rotationalmechanism.

SUMMARY OF THE INVENTION

A reflective afocal switching assembly permits variable fields of viewwhile at the same time providing a common axis and mechanism to achievean optical de-roll of the image. This complex arrangement provides arelatively large change in magnification for an all-reflective opticalsystem than can image over 0.4-12.0 micron spectrum.

A method and apparatus involving co-location of a high magnificationoff-axis reflective afocal with a lower magnification power afocal isdisclosed, wherein the low power afocal as a subassembly rotates 90°about an common roll axis within the high power afocal to eitherintercept or bypass the optical beam and thereby change the systemmagnification, while at the same time both assemblies may rotatetogether about the same exit pupil optical axis by use of a commonmechanism in order to produce image roll. To achieve a thirdmagnification option, a flat mirror may also be inserted along theoptical axis to bypass both the high and low power afocals, resulting ina simple unity 1× power optical relay which also maintains rotationabout a common roll axis.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional advantages and features will become apparent as the subjectinvention becomes better understood by reference to the followingdetailed description when considered in conjunction with theaccompanying drawings wherein:

FIG. 1 illustrates a high magnification mode of operation in which lightrays are shown entering an exemplary multiple field of view off-axisreflective afocal assembly with common axis from an outside scene.

FIG. 2 shows the positions of mirror elements and associated mountingfeatures rotated 90° about an exit pupil such that the light rays whichoperate in the high magnification mode are not obscured.

FIG. 3 illustrates a low magnification mode of operation in which a lowpower afocal optical assembly is now rotated 90° about the axis of theexit pupil in order to intercept a different set of light rays from thescene.

FIG. 4 shows a superimposed trace of both afocal assemblies relative tothe common roll axis.

DETAILED DESCRIPTION

FIG. 1 illustrates a high magnification mode of operation in which lightrays are shown entering an exemplary multiple field of view off-axisreflective afocal assembly with common axis from an outside scene.Specifically, FIG. 1 shows the high magnification mode in which opticallight rays 1 enter the optical system from an outside scene. In the highpower afocal mode of operation, the light rays 1 strike the afocalprimary mirror 2, then propagate to secondary and tertiary mirrors 3 and4, and then progress to a final fold mirror 5 which directs lightthrough the exit pupil 6 which defines the common roll axis. The primarymirror 2 is concave to provide positive optical power and may have aparabolic, conic, or higher order aspheric curvature. The secondarymirror 3 is has a negative convex shape to supply negative opticalpower, and it may be hyperboloid, conic, or a higher order asphere. Thetertiary mirror 4 has a basic concave surface to provide positiveoptical power, and it may have parabolic, conic, or higher orderaspheric surface curvatures. The fold mirror 5 is optically flat andserves to re-direct the optical beam towards the system exit pupil 6.Mirror elements 8, 9, 10, and 11 comprise the low power afocal and arebypassed in this mode, as they are rotated 90° about the common rollaxis 12. The low power assembly has elements similar to that of the highpower, but in a slightly different order. Element 8 is a concave primarymirror, mirror 9 is a negative powered secondary. Mirror 10 simply foldsthe system, and has a flat surface with virtually no optical power.Mirror 11 serves as the tertiary in terms of providing the finalpositive power. Note that the function of mirror 10 in the low powerconfiguration is analogous to mirror 5 in the high power configuration,except that in the low power configuration the mirror 10 is locatedbetween the secondary 9 and tertiary 11, whereas in the high powerassembly the flat mirror 5 is the last element prior to the exit pupil.This shift in the relative location of the flat fold mirrors facilitatesthe common de-roll axis. Generally, the above mirrors can all bemachined using single point diamond turning techniques which provide thenecessary surface figures and specular smooth finishes. The mirrors aretypically made of readily available metals, such as Aluminum 6061, inorder to provide thermal equilibrium when the housings are made ofsimilar materials. In the embodiment shown, the high power afocal systemprovides an 8× magnification with an 8″ entrance pupil, a minimum 1°field of view, and a 1″ exit pupil. The low power afocal system providesroughly 3.2× magnification, a 3.2″ entrance pupil, a minimum 3° field ofview, and a matching 1″ exit pupil.

FIG. 2 shows the positions of mirror elements and associated mountingfeatures rotated 90° about an exit pupil such that the light rays whichoperate in the high magnification mode are not obscured. Specifically,FIG. 2 shows the positions of the mirror elements 8, 9, 10, and 11 andany associated mounting features is such that when rotated 90° about theexit pupil axis 12, they do not obscure the light rays 1 which operatein the high magnification mode.

FIG. 3 illustrates a low magnification mode of operation in which a lowpower afocal optical assembly is now rotated 90° about the axis of theexit pupil in order to intercept a different set of light rays from thescene. Specifically, FIG. 3 represents the low magnification mode,wherein the low power afocal optical assembly comprised of mirrors 8, 9,10, and 11 is now rotated 90° about the axis 12 of the exit pupil 6relative to FIG. 1 in order to intercept a different set of light raysfrom the scene 7, bounce off the mirrors 8, 9, 10, and 11. A third fieldof view option for a total 1× magnification can be obtained by flippingon a flat mirror 13 between the low power afocal tertiary mirror 11 andthe exit pupil 6.

FIG. 4 shows a superimposed trace of both afocal assemblies relative tothe common roll axis 12. Either afocal assembly may rotate about 12 inorder to achieve an image de-roll effect. In this way, the samemechanism used to switch between afocals can also be used for de-roll.

It is obvious that many modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as described.

What is claimed is:
 1. A high magnification mode of operation of amultiple field of view optical system based on off-axis reflectiveafocal assemblies having a common axis, the steps of the highmagnification mode of operation comprising: disposing a highmagnification mode off-axis reflective afocal assembly rotatable about acommon axis to be able to receive optical light rays entering themultiple field of view optical system from an outside scene such that:an afocal primary mirror is oriented such that the optical light raysreflect from a concave reflective surface of the afocal primary mirroras first reflected light rays, the first reflected light rays thenpropagate to reflect from a negative convex shape reflective surface ofa secondary mirror as second reflected light rays, the second reflectlight rays then are reflected from a basic concave surface of a tertiarymirror as third reflected light rays, and the third reflected light raysthen progress to a final fold mirror which redirects the third reflectedlight rays as a high magnification light through an exit pupil whichdefines the common axis, wherein fold mirror is optically flat andserves to re-direct the optical beam towards the system's exit pupil;and rotating a low magnification mode reflective afocal subassemblyabout the common axis such that the low magnification mode reflectiveafocal subassembly does not optically interfere with the highmagnification mode of operation, wherein, either of the highmagnification mode off-axis reflective afocal assembly and the lowmagnification mode reflective afocal subassembly can each rotate aboutthe common axis to switch the multiple field of view optical systembetween high or low magnification modes, or to achieve image de-roll. 2.The high magnification mode of operation of a multiple field of viewoptical system according to claim 1, wherein the low magnification modereflective afocal subassembly is rotated 90° about the common axis suchthat the low magnification mode reflective afocal subassembly as rotateddoes not obscure optical paths of the high magnification mode off-axisreflective afocal assembly.
 3. The high magnification mode of operationof a multiple field of view optical system according to claim 1, whereinthe concave reflective surface of the afocal primary mirror providepositive optical power and can have a parabolic, conic, or higher orderaspheric curvature.
 4. The high magnification mode of operation of amultiple field of view optical system according to claim 1, wherein thenegative convex shape reflective surface of the secondary mirrorsupplies negative optical power, and is hyperboloid, conic, or a higherorder asphere.
 5. The high magnification mode of operation of a multiplefield of view optical system according to claim 1, wherein the basicconcave surface of the tertiary mirror provides positive optical power,and can have parabolic, conic, or higher order aspheric surfacecurvatures.
 6. The high magnification mode of operation of a multiplefield of view optical system according to claim 1, wherein, the finalfold mirror redirects the third reflected light rays as a highmagnification light to provide an output of 8× magnification with an 8″entrance pupil, a minimum 1° field of view, and a 1″ exit pupil.
 7. Alow magnification mode of operation of a multiple field of view opticalsystem based on off-axis reflective afocal assemblies having a commonaxis, the steps of the low magnification mode of operation comprising:Rotating a low magnification mode reflective afocal subassembly about acommon axis of an exit pupil relative to a high magnification modeoff-axis reflective afocal assembly in order to intercept a differentset of light rays from a scene such that: a concave primary mirror isoriented such that the different set of light rays from a scene reflectsfrom a concave reflective surface of the concave primary mirror as firstreflected light rays, the first reflected light rays then propagate toreflect from a negative powered secondary mirror as second reflectedlight rays, the second reflected light rays then are reflected from aflat surface mirror as third reflected light rays, and the thirdreflected light rays then is reflected from a low power afocal tertiarymirror as a low magnification light, which is directed along the commonaxis to the exit pupil defining the common axis.
 8. The lowmagnification mode of operation of a multiple field of view opticalsystem according to claim 7, the steps of the low magnification mode ofoperation further comprising: flipping on a flat mirror between the lowpower afocal tertiary mirror and the exit pupil to obtain a third fieldof view option for a total 1× magnification.
 9. The low magnificationmode of operation of a multiple field of view optical system accordingto claim 7, wherein the flat surface mirror is disposed between thenegative powered secondary mirror and the low power afocal tertiarymirror to facilitate a common de-roll axis.
 10. The low magnificationmode of operation of a multiple field of view optical system accordingto claim 7, wherein either of the high magnification mode off-axisreflective afocal assembly or the low magnification mode reflectiveafocal subassembly may rotate about the common axis order to achieve animage de-roll effect, whereby the multiple field of view optical systemis capable of switching between afocals and for de-roll.
 11. The lowmagnification mode of operation of a multiple field of view opticalsystem according to claim 7, wherein the low power afocal tertiarymirror provides final positive power to the low magnification light toresult in an output of 3.2× magnification, a 3.2″ entrance pupil, aminimum 3° field of view, and a matching 1″ exit pupil.